Protective diffusion layers are particularly advantageous for gas turbine engine components. Such components are subject to high temperatures and oxidizing and hot corrosive environments. Further, many of these parts have complex and intricate designs.
It is already known that aluminide coatings increase the life expectancy of parts made with high temperature alloys when they are exposed to oxidation and corrosion. Currently, these coatings are produced principally by pack cementation and gas-phase chemical vapor deposition. For example, it is known to apply a diffusion layer of aluminum in nickel, cobalt and iron-base alloy parts by pack cementation processes. These involve packing such parts in a bed of powdered mixture consisting of a source of aluminum and an inert filler material and heated to an elevated temperature, for example, 750.degree. to 1100.degree. C., for several hours to diffuse and build up a layer of aluminum on the surfaces of the alloy parts being treated. See U.S. Pat. No. 4,156,042 and K. Brennfleck, PROC. INTERN. CONFERENCE ON CVD, p. 578 (1979).
Regarding more complex structures, U.S. Pat. No. 4,148,275 teaches to diffusion aluminize hollow tubes or the like using a CVD process by connecting the hollow portions to a manifold and to force a carrier gas over a heated bed of a mixture of a source of aluminum and an inert filler and into the hollow portions to carry a portion of volatilized aluminum into the passages.
An alternative CVD method for coating internal channels is the use of organo-metallics as the vapor transport species. Experimental work with aluminum alkyls has shown that it is capable of satisfactorily coating the walls of channels down to a diameter of 0.25 mm (see J. E. Restall in ALTERNATIVE PROCESSES AND TREATMENTS, Mat. Sci. Technol. 2, 226-231 (1986).
Ion Vapor Deposition (IVD) is a process combining vacuum evaporation and sputtering to also achieve such coatings. This known process is described in G. Rudzki, SURFACE FINISHING SYSTEMS, p. 161 (1983).
Electroplating is an electrochemical process conducted in a solution resulting in the deposition of a coating on an electrode (see G. Rudzki, Surface Finishing Systems, p. 45 (1983)).
Electrophoretic deposition, which is analogous to electroplating involves the coagulation of resins on the electrode and can be employed to deposit aluminum, as described by T. P. Fisher, Surface Technology, 12, 107 (1981).
It is equally known that the presence of transition metals such as platinum and yttrium is useful. Nevertheless, to date, there are a few methods of applying transition metals to the parts. For instance, CVD from noble metal halides is not efficient due to the proximity of the volatilization and decomposition temperatures (see C. F. Powell in VAPOR DEPOSITION, John Wiley and Sons, New York, p. 310 (1966)). Equally, very few studies of metal-organic deposition have been reported, largely because of the lack of suitable volatile and reactive molecular precursors capable of producing uncontaminated films (see M. Rand, CHEMICAL VAPOR DEPOSITION OF THIN-FILM PLATINUM, J. Electrochem. Soc., 120, 686-693, (1973) and J. Gozum, TAILORED ORGANOMETALLICS AS PRECURSORS FOR THIN CHEMICAL VAPOR DEPOSITION OF HIGH-PURITY PALLADIUM AND PLATINUM FILMS, J. Am. Chem. Soc., 110, 2688-2689 (1988).
The most common method of platinum deposition is electroplating. In a preferred method, the platinum group metal is applied first, and is diffused at the same time as the aluminum is deposited. Vapor deposition is a technique listed but not exemplified to deposit the platinum group metal. Rather, electro-deposition was exemplified. See British Patent No. 1,210,026 and U.S. Pat. No. 4,501,776.
Similarly, U.S. Pat. No. 3,677,789 proposes to improve the oxidation and corrosion resistance of such articles by first coating the alloy part with a platinum group metal by electro-deposition or other means. Then, the patent teaches to aluminize the platinum plated part by pack cementation.
Recently, a method has been discovered for forming protective diffusion layers on nickel, cobalt and iron-base alloy parts. It comprises the formation of a diffusion layer of platinum, chromium and aluminum on surfaces, either by deposition of platinum and gas phase chrominizing followed by aluminizing or gas phase chrominizing and deposition of platinum followed by aluminizing and deposition of platinum. Simply, this method is directed to a sandwich structure which includes a chromium layer as an intermediate to facilitate the adhesion of a platinum layer to aluminum. Electro-deposition of platinum is a preferred technique, although vapor deposition is mentioned but not exemplified. See U.S. Pat. No. 4,526,814.
Chemical vapor deposition is a process of obtaining an element in solid form by decomposition of a gaseous compound. This is radically different from the physical process of vapor deposition, which is the physical process of condensing elemental vapors. While electroplating and vapor deposition are line-of-sight processes, chemical vapor deposition permits coating of complex structures including the cooling internal passages of the blades. It would be desirable to deposit transition metals to obtain modified coatings into these passages.
Recently, R. Kumar, NEW PRECURSORS FOR CHEMICAL VAPOR DEPOSITION OF PLATINUM AND THE HYDROGEN EFFECT ON CVD, polyhedron, 551 (1988) and H. D. Kaesz, LOW TEMPERATURE OMCVD FOR PLATINUM, Adv. Coat. Surf. Tech., 2,3 (1989) have been able to deposit clean platinum films utilizing new precursors that decompose under mild conditions; the presence of hydrogen in order to obtain pure films is emphasized by the authors.
Fortunately, applicants have discovered a method which overcomes the deficiencies of prior art in preparing modified aluminide coatings on complicated structures and in coating platinum or a transition metal directly on aluminum or aluminide without the necessity of using intermediates to facilitate such bonding.